I was having some knee problems when riding my recumbent bike.
So I did some googling and asking around.

Googling told me that it’s best to pedal 90RPM, but not why.
Asking around on the Dutch ligfiets group produced a long thread of good info.
This post is a collection of that information.

Before I start, I’ll tell you to go see a physiotherapist.
Reading things on the internet is not a substitute for professional care.
You only have one pair of knees, and I’ve been told cartilage never grows back.

The problem

Gravity, imago, and wiggle room.

On a normal bike your power is limited to your weight.
On a recumbent it’s only limited by your muscles.
This means you can push harder than is good for you.

Recumbents have the imago of being fast.
So you will push harder than is good for you.

On a recumbent you don’t get “gravity asist”.
On a normal bike you can cruise along by just lifting and dropping your legs.
This doesn’t work so well horizontally.

People are asymetric, but bikes are not.
On a normal bike your hips accomodate this asymetry.
On a recumbent your hips are less free.

Why 90RPM

The end goal is reducing strain on your knees,
so it is obvious that a lighter gear means less strain.

In this regard the human body is like a combustion engine.
There is a speed at which it produces the most power,
and a speed where it is the most efficient.

90RPM is the speed where you can produce the most power.
60RPM is the speed where you are the most efficient.
Approximately, depending on you and your bike.

Rules of thumb

Keep a constant pace, vary your speed and gear.

Don’t fight headwind and hills. Reduce gear.

Shift to first gear when stopping.

Reduce gear when slowing down.

Accelerate slowly.

First accelerate, then gear up.

Keep your speed at least above 60RPM.

Pedal lighly, ride slowly.

How much is 90RPM

Three half revolutions per second.
Not that much when you get used to it.

A cheap way to find out is to install a metronome app on your phone, and set it to 90bpm.
Now make a whole revolution per 1/4, or one leg per 1/8, assuming a 4/4 measure.

If you’re doing it right it’ll feel like you’re just moving your legs,
instead of pushing the bike.

Once you get a feeling for it, let your mind go.
I cycle much more naturally and relaxed when I’m not watching every move.
It’s not helpful to stick to exactly 90RPM in a non-constant environment.
Your body will interrupt you when conditions change significantly.

Other helpful things

Adjust the lenght of your bike real precise.
It can be a matter of millimeters, due to the limited hip movement mentioned earlier.
Too long and you knees will over-strech, too short and you push less efficiently.
From what I heard, the former manifests as pain in the back of your knee,
the latter as pain in the kneecap.

Some people report that shorter cranks make it easier to pedal fast.
This might be especially true if you’re short.

Some people use oval gears, claiming they provide a more constant feeling.
The idea is that the gear is small at the dead spot and large when perpendicular to your legs.

Consider SPD pedals and shoes.
At a high RPM a lot of energy goes towards keeping your feet on the pedals.
Using SPD pedals make it easy to pedal fast, and as a bonus allow you to pull as well as push.